A radio intended for two-way communication generally operates with either an external fixed rod or retractable antenna, or with an internal antenna. The fixed rod type of antenna has a predetermined length. Whilst such antennas can be relatively short, they are not conductive to a compact design nor are they particularly suitable for a radio intended to be carried in a pocket or other receptacle offering restricted space. On the other hand, retractable antennas are convenient for this purpose because they can be folded away when the radio is not in use. Retractable antennas are commonly of the telescopic tube type, although retractable fixed length antennas are also known.
Some known portable radios such as that disclosed in U.S. Pat. No. 3,087,117 have two antennas, i.e. an internal element together with a retractable element, and are also equipped with means for automatically switching between the two elements according to the physical position of the retractable element. Hence the retractable antenna is operable in the extended position, while the internal antenna element becomes operable when the retractable element is in the retracted position.
An important consideration with a dual antenna system is that both antennas should provide efficient operation under different conditions as appropriate. For example, while the external antenna element may provide better sensitivity and range performance during normal operation, the less efficient internal antenna must provide satisfactory performance during stand-by operation.
U.S. Pat. No. 4,868,576 discloses an antenna for a portable cellular telephone comprising a helical coil at the base of a retractable elongate radiating element. The retractable element, which extends through the helical coil, has non-conductive portions at its two ends whereby in the extended position the elongate element is capacitively coupled to the helical coil, and in the retracted position the elongate element is substantially decoupled therefrom. The helical coil is fixedly mounted on the housing of the radio transceiver.
British patent GB-A-844,968 discloses an arrangement for connecting an external antenna to a radio receiver having a single existing, internal antenna intended for use in a vehicle. A housing is provided into which the radio is slidably mounted. A jack plug connected to the external antenna is fixedly mounted on the rear wall of the housing so that when the radio is fully inserted into the housing the jack plug mates with an antenna socket provided in the back of the radio apparatus. The housing acts to screen the internal antenna and thereby render it inoperative when the jack plug and hence the external antenna are connected. This arrangement requires the radio to be almost completely encased within the housing, so making the radio relatively inaccessible when connected to the external antenna. Also, since the internal antenna remains physically and electrically connected to the radio circuits, this arrangement is not suitable for a radio intended for transmission as well as reception.
EP-A-0,343,848 discloses an antenna assembly for enabling an external antenna to be connected to a portable radio in place of an existing antenna. An adapter is slidably located in a passageway, which may be part of an integral moulding of the radio casing. The existing antenna is slidably located in the same passageway such that it is displaced and so disconnected from the radio when the adapter is fitted. Alternatively, the radio has an internal existing antenna which is switched out when the adapter makes an external antenna connection.